1. Field of the Invention
The present invention is directed to a switchable longitudinal gradient coil of the type having a cylindrical structure and windings that are symmetrically arranged around the longitudinal axis.
2. Description of the Prior Art
The required efficiency of a gradient coil is essentially dependent on the type of the MR-imaging. Conventional MR-imaging normally requires a good linearity volume (xcx9c5% in the linearity volume of 40-50 cm) given moderate gradient intensities (10-20 mT/m) and switching times (xcx9c1 ms). High gradients (20-40 mT/m) are extremely rapidly switched (100-500 xcexcs) for the fast MR-imaging. Side effects in the form of peripheral muscle stimulations in the patient disposed in the field can arise as a result. Generally, the linearity volume of the gradient coils is reduced in order to avoid these effects; this leads to a reduction of the maximal field boosts and therefore also reduces the stimulation risk. The maximal field boost, in addition to other factors, determines the stimulation risk. Therefore, the linearity volume can become reduced from typically 40-50 cm to 20 cm DSV given fast gradient coils. A coil with such properties is normally not appropriate for conventional whole body applications, but is appropriate for fast MR-imaging, such as EPI, RARE, HASTE, GRASE etc. The speed is the essential advantage in such sequences.
Another reason for different field qualities is that the linearity normally diminishes with the distance from the center when a gradient coil is designed for a specific volume. The human body does not necessarily follow this. For example, the shoulders are situated in this area. Given imaging of the spine, it is often expedient to image the entire spine without rearrangement of the patient. Depending on the center positioning, the cervical and/or lumbar vertebras also lie in the area of the larger non-linearities. Image distortions cannot be avoided as a result. Given head gradient coils, the homogeneity volume is smaller due to the smaller diameter of the coil. This only allows imaging parts of the brain but does not allow the imaging of the cervical spine. Therefore, it can be desirable for the radiologist to change from a central FOV to a displaced or shifted FOV, but this has hitherto not been possible. There are only coils of the one type or the other type.
Due to the above reasons, the customer (apparatus purchaser) therefore has to decide whether he wants to have a field quality A, B or C. However, it would be desirable to have a number of coil properties (field qualities) united in one coil and to be able to activate these depending on the application. A basic problem is the accommodation of the multiple coils in the coil body without significantly increasing the volume (resulting in the coils becoming more expensive) and compromising the sub-coil properties that are partially competing. xe2x80x9cLongitudinalxe2x80x9d gradient coils are known in the field of magnetic resonance imaging and are gradient coils that provide a gradient field in the direction of the basic magnetic field, namely in the direction of the cylinder axis, given cylindrical magnets. Since this direction is also referred to as the xe2x80x9cz-directionxe2x80x9d in the MR-technique, the longitudinal gradient coils are also referred to as xe2x80x9cZ-gradient coilsxe2x80x9d in the following.
German OS 195 40 746 describes a modular gradient system in which a conventional and a fast gradient coil system are united in a coil body. The conventional gradient coil system exhibits a large linearity volume but can be switched only slowly and also allows only average gradient amplitudes. In contrast thereto, the fast gradient system has a smaller linearity volume but allows the faster switching of extremely high gradient amplitudes.
Whereas a series of possibilities have already been proposed for the transversal coils, which are usually four in number and are referred to as saddle coils, in order to generate a gradient field that is designed according to the required performance features, conventional Z-gradient coils do not allow this. These Z-gradient coils or longitudinal coils are normally composed of a number of solenoid sub-coils that are symmetrically disposed around the Z=0 position. Both halves Z greater than 0 and Z less than 0 are switched such that the halves are operated with respectively opposite current directions, so that a linear gradient field arises in total.
An object of the present invention is to provide a switchable Z-gradient coil that allows a versatile changing of the magnetic field generated with the coil, which achieves a better adaptation to the required imaging conditions without physically re-orienting the coil.
This object is inventively achieved in a Z-gradient coil composed of discrete windings or winding packets or of continuous windings or continuous winding packets, which are provided with separate supply lines to an end face and which, outside of the coil, can be individually connected to one another and/or can be connected to one another in groups, depending on the desired performance features.
In the extreme case, an inventive switchable Z-coil is composed solely of open single windings, whose respective winding ends are separately led outside by means of supply lines, so that the single windings can be arbitrarily connected with one another to different sub-groups that enable respectively different performance features. The selectable performance features are the linearity, the linearity volume, the shielding, the inductivity, the noise (participation factors), the stimulation sensitivity, the maximum gradient intensity and the maximum slew rate.
The ability to reverse the current direction is also one of the different optional connecting possibilities of the individual windings and winding packets. Among other things, this enables a connectable Lorentz force compensation.
The circuit arrangement for the connecting of the windings and winding packets with one another, as desired, so as to correspond to the desired performance features; can be designed such that the performance features are statically fixed and connected before a sequence operation or such that the performance features are dynamically switchable during a sequence operation.
The individual windings can exhibit different diameters, with the windings and winding groups of the respectively radial planes being connectable to units, such as primary coils and secondary coils.